The present invention is directed to apparatus for perforating a wellbore. More particularly, the present invention is directed to perforating apparatus that can be run in through the production tubing.
When perforating a wellbore, the use of an underbalanced technique (i.e., without the use of high density muds to control, or overbalance, the pressure of the wellbore fluids) has certain advantages. For example, when perforating is completed, the rush of produced fluids into the wellbore will clean out the newly formed perforations. In an overbalanced method, the combination of high density drilling fluid and surface pressure not only risks fracturing the reservoir, thereby permitting the trapped fluids to escape, but can also block off the perforations.
In the underbalanced perforating method, a packer and production tubing are run in the hole first to permit flow control of produced fluids. Accordingly, two types of underbalanced perforating apparatus are available: tubingconveyed perforating tools and through-tubing perforating tools. Through-tubing charges are necessarily small and, therefore, should be shaped to project maximum energy into the formation and efforts should be made to minimize dissipation of that energy. These efforts could include orienting the charges to fire laterally into the formation, not placing any superfluous structure between the charge and the formation, and placing the charge immediately proximate the casing wall. Some prior art perforating tools hold the through-tubing charges against one side of the borehole wall. However, such a perforating pattern known as 0.degree. phasing can reduce produced fluids by as much as 10% when compared to 180.degree. phasing (alternate charges actuatable into opposite sides of the formation).
The present tool allows 180.degree. phasing with maximum formation penetration because the charges are pressed firmly against opposite sides of the borehole walls. Individual shaped charges are interconnected by rigid linkage means. The linkages are of such a length that twice the length of the charges extending beyond the connecting points plus the length of the linkage exceeds the diameter of the borehole causing the charges to be pressed firmly into contact with the wellbore. Longer linkage lengths may be used to reduce charge density. The configuration of the perforating apparatus is such that the maximum formation penetration possible will be achieved by projecting the full force of the charge in a direction perpendicular to the wall of the wellbore. Additional charges can be easily added as circumstances warrant. The expended charges can be released from the wireline and permitted to fall into the bottom of the wellbore (i.e., the rathole). A sinker bar can be used to assist in conveying the perforating apparatus through the production tubing and in assuring that the expended device falls into the rathole.
Other features, advantages and characteristics of the present invention will become apparent after a reading of the following detailed description.